The present invention relates generally to resistance welding. More particularly it relates to the resistance welding of two steel members having respective compositions each comprising at least one alloying element having a concentration which contributes substantially to solidification cracking extensive enough to produce an unacceptable weld fracture.
In resistance welding, two steel members are welded together typically by positioning a first portion of one steel member in superimposed relation to a first portion of the other steel member, at a welding station where a current is flowed through the two superimposed portions of the steel members to fuse those portions together, thereby creating a weldment or weld nugget. The flow of current through the two steel members causes melting at the first portions of these two steel members, at the location of the weldment, and the metal at the weldment remains molten until the flow of current is reduced or ceases, following which solidification occurs at the location of the weldment.
During solidification, a phenomenon known as solidification cracking can occur in the weldment; solidification cracking can be extensive enough to produce an unacceptable weld fracture during a test known as chisel testing. In chisel testing, a hammer and chisel are used to forcibly separate the two steel members at the weld location. The resulting weld fracture can be (a) either wholly or partially through the weldment, or the fracture can be (b) in the first portion of one of the two steel members, in a so-called "heat-affected zone" immediately surrounding the weldment, with the fracture manifesting itself as a hole in the heat-affected zone. The type of fracture described at (a) in the preceding sentence is an unacceptable weld fracture while the type of fracture described at (b) is acceptable.
Solidification cracking occurs in the following manner. Cooling and solidification of the molten metal at a weld location occurs directionally from the periphery inwardly and is in the form of finger-shaped solidification cells or dendrites. Between adjacent dendrites, there is a continuous film of molten metal having a lower solidification temperature than the metal in the solidified dendrites, due to segregation between the dendrites of molten metal having a higher concentration of alloying elements such as carbon, manganese and the like. As further cooling occurs to a temperature below the solidification temperature of the segregated metal between the adjacent dendrites, voids are created by the synchronous application of solidification strain on the solidifying dendrites which pulls the dendrites apart before they can cohere with adjacent dendrites. The greater the concentration of an alloying element which segregates in the manner described above, the greater the contribution of that element to solidification cracking; and the greater the extent of solidification cracking, the greater the likelihood that there will be an unacceptable weld fracture.
There is a first group of steel compositions which, when used in two steel members which are to be resistance welded solely to each other, produce solidification cracking in the weldment extensive enough to produce an unacceptable weld fracture; and for this group, there are no commercially practical modifications to the welding schedule (welding parameters) which can be used to overcome this weld defect. In the case of this first group, the steel compositions must be replaced with other compositions more amenable to resistance welding. A drawback to such replacements is that the replacing steel compositions usually have physical or mechanical properties inferior to those of the steel compositions which are being replaced.
There is a second group of steel compositions which, when used in steel members which are to be resistance welded solely to each other, similarly produce extensive solidification cracking and unacceptable weld fractures; however, for this group of steel compositions, modifications to the welding schedule can often be employed to overcome the problem. Modifications to the welding schedule, however, usually result in increased expenditure of time, effort and money and a slower and less efficient welding operation.